1. Field of the Invention
The present invention concerns the general structure of satellites, for example observation satellites and in particular Earth observation satellites. It is particularly directed to spin-stabilized satellites; it is particularly, but not exclusively, concerned with meteorological satellites.
2. Description of the Prior Art
Spin-stabilized geostationary meteorological satellites have already been used by the Unites States, the Soviet Union, Japan and Europe as part of the WWW (World Weather Watch) meterological observation program. Such satellites include the US GOES 1 and 4 satellites, the European MOP satellite and the Japanese GOIS satellite.
Until now terrestrial observation satellites have always been spin-stablized and have always used solid propellant apogee maneuver systems.
At the present time various projects are in hand, in particular in the United States and Europe, towards launching second generation satellites, so called to distinguish them from existing (or first generation) satellites; these second generation satellites are intended to have significantly better performance than first generation satellites, especially with regard to the number of channels and pointing accuracy; this implies a dry mass much greater than that of first generation satellites (some 800 to 1 500 kg, as compared with approximately 300 to 350 kg previously).
Since the time when the first generation satellites were designed and launched, important changes have occurred in the field of satellite propulsion, applicable to geostationary and other type satellites: the use of solid propellant apogee motors has given way to the use of liquid propellant propulsion systems.
This general adoption of liquid propellants as opposed to solid propellants is explained by their better specific impulse, the possibility of providing a unified propellant system for the apogee maneuver, orbit correction and attitude control systems, increased operational flexibility (the facility to carry out an apogee maneuver in a number of phases) and a more flexible concept (the liquid propellant tanks are filled immediately before launch to the maximum allowable mass of the satellite, whereas previously it would have been necessary to choose the solid propellant motor and its propellant tank at some stage during the development process on the basis of the projected mass of the completed satellite). The use of liquid propellants results in a significant savings of mass at launch.
One consequence of this trend is that there is not available in Europe at this time any flight-qualified solid propellant apogee motor suitable for the masses typical of modern satellites (800 to 1 500 kg dry mass).
Also, the use of liquid propellant apogee thrusters poses a particular and critical problem in the case of observation satellites (especially meteorology satellites) that are spin-stabilized.
The role of observation satellites entails imaging the Earth and/or its atmosphere in the infra-red band. The sensors used for this have to be cooled to low temperatures, in the order of 100K, to achieve acceptable signal/noise characteristics. These low temperatures are conventionally obtained by placing the focal plane of the observation instrument (on which the infra-red sensors are located) under a frustoconical radiator disposed on a transverse face of the satellite facing towards deep space, in order to minimize the flow of energy, especially solar energy, from the exterior and prejudicial to the removal of heat by the radiator. The frustoconical side wall of the radiator is conventionally inclined at an angle slightly greater than 23.5.degree. to a plane transverse to the spin rotation axis and is highly polished (it is usually made from aluminum) to make it highly reflective and to reject outside the radiator any incident solar radiation, even under worst case conditions (in particular, the winter solstice when the radiator is on the SOUTH face of the satellite).
In all first generation satellites the field of view of the passive radiator towards deep space (towards the SOUTH) was achieved by jettisoning the solid propellant apogee motor after the burn; the radiator was situated immediately behind the apogee motor, at the satellite/apogee motor interface. From this point of view the solid propellant apogee motor has the advantage of constituting with its propellant tank a compact and easily jettisoned assembly.
The opposite (NORTH) face of the satellite is occupied by the ground communication antennas which include a telecommunication boom disposed accurately along the spin rotation axis of the satellite.
Jettisoning the apogee maneuver system appears to be out of the question in the case of a liquid propellant system (especially with a unified propellant system), given that the propellant for the apogee burn is fed to the apogee thruster (which is accurately oriented along the spin rotation axis) from storage tanks inside the body of the satellite by means of pipes and that it is not feasible, for reasons connected with sealing, to provide a break between the storage tanks and the apogee thruster to allow the latter to be jettisoned.
Designing spin-stabilized geostationary observation satellites, therefore, requires a solution to be found to the following technical problem: how to install the axially disposed antennas, the axially disposed liquid propellant apogee maneuver system and the axially disposed radiator of the observation system while simultaneously meeting constraints associated with spin rotation of the satellite (in particular, the observation system conventionally includes mobile optical parts which it has seemed essential to keep as close as possible to the spin rotation axis to protect them from excessive centrifugal forces). On a more general level, the problem is to locate on the rotation axis of a spin-stabilized satellite an axial propulsion system and two axial equipments (in this instance an antenna boom and a radiator).
More generally, the problem is to accommodate on a spin-stabilized satellite an axial propulsion system, a set of communication antennas and an equipment which must, like the set of antennas, be disposed along the spin rotation axis.